encoder/filename: Add SCSU as tables

Instead of only adding SCSU, add it as an existing table.

Allow direct SCSU and add a, perhaps, reasonable table as well.

Add byte interfaces that doesn't base64 encode the URL as well with `EncodeBytes` and `DecodeBytes`.

Fuzz tested and decode tests added.

lib/encoder/filename/encode.go

@@ -4,6 +4,7 @@ import (
 	"encoding/base64"
 	"encoding/binary"
 
+	"github.com/dop251/scsu"
 	"github.com/klauspost/compress/huff0"
 )
 
@@ -11,21 +12,45 @@ import (
 // Calling Decode with the returned string should always succeed.
 // It is not a requirement that the input string is valid utf-8.
 func Encode(s string) string {
+	table, payload := EncodeBytes(s)
+	return string(encodeURL[table]) + base64.URLEncoding.EncodeToString(payload)
+}
+
+// EncodeBytes will compress the given string and return a table identifier and a payload.
+func EncodeBytes(s string) (table byte, payload []byte) {
 	initCoders()
 	bestSize := len(s)
-	bestTable := tableUncompressed
+	bestTable := byte(tableUncompressed)
 	org := []byte(s)
 	bestOut := []byte(s)
-
 	// Try all tables and choose the best
 	for i, enc := range encTables[:] {
+		org := org
 		if len(org) <= 1 || len(org) > maxLength {
 			// Use the uncompressed
 			break
 		}
+
 		if enc == nil {
 			continue
 		}
+
+		if i == tableSCSU {
+			var err error
+			olen := len(org)
+			org, err = scsu.EncodeStrict(s, make([]byte, 0, len(org)))
+			if err != nil || olen <= len(org) {
+				continue
+			}
+			if len(org) < bestSize {
+				// This is already better, store so we can use if the table cannot.
+				bestOut = bestOut[:len(org)]
+				bestTable = tableSCSUPlain
+				bestSize = len(org)
+				copy(bestOut, org)
+			}
+		}
+
 		// Try to encode using table.
 		err := func() error {
 			encTableLocks[i].Lock()
@@ -36,14 +61,14 @@ func Encode(s string) string {
 			}
 			if len(out) < bestSize {
 				bestOut = bestOut[:len(out)]
-				bestTable = i
+				bestTable = byte(i)
 				bestSize = len(out)
 				copy(bestOut, out)
 			}
 			return nil
 		}()
 		// If input is a single byte repeated store as RLE or save uncompressed.
-		if err == huff0.ErrUseRLE {
+		if err == huff0.ErrUseRLE && i != tableSCSU {
 			if len(org) > 2 {
 				// Encode as one byte repeated since it will be smaller than uncompressed.
 				n := binary.PutUvarint(bestOut, uint64(len(org)))
@@ -56,5 +81,5 @@ func Encode(s string) string {
 		}
 	}
 
-	return string(encodeURL[bestTable]) + base64.URLEncoding.EncodeToString(bestOut)
+	return bestTable, bestOut
 }